The invention applies, for example, but not restrictively to calibrating the gain of a voltage-controlled oscillator (VCO) in phase-locked loops (PLL).
A phase-locked loop is a looped system capable of delivering a signal where the frequency is slaved to that of a reference signal.
More precisely and conventionally, a phase-locked loop notably includes a phase comparator receiving on one input a reference signal, and a voltage-controlled oscillator where the control input is coupled to the output of the phase comparator and where the output, looped back onto another input of the phase comparator after a frequency division, delivers an output signal slaved in frequency and phase to that of the reference signal but generally having a higher frequency than that of the reference signal.
During a calibration in gain of the voltage-controlled oscillator, the frequency of the oscillator output signal is determined for two different values of the input voltage (generally obtained from two different digital words respectively representative of the two different voltages), and the ratio is obtained between the difference of these two output frequencies and the difference of the two digital words.
Methods are known for calculating a frequency of a signal from the frequency of a reference signal, notably using an average of the numbers of periods of the first signal counted during a given number of periods of the reference signal. However, the more accurate the measurements have to be, the more time-consuming these methods are.
By way of example, for a measurement accuracy of 62.5 kHz, the accurate determination of the frequency of a signal which is, for example, around 2.4 GHz, from a reference signal having a frequency of 16 MHz requires 256 periods of the reference signal, or 16 microseconds.
This leads to a substantial duration of the gain calibration phase.
However, some low consumption communication applications, such as Bluetooth low energy (BLE), require a faster start of the phase-locked loop, and therefore in particular a short duration of the gain calibration phase of the oscillator notably in order to save battery power.
Thus, according to one implementation and embodiment provision is made for a simple and fast determination of a frequency of a signal from a reference signal.